1. Field of the Invention
The present invention relates to semiconductor manufacturing, and more particularly to controlling the leveling (upper) table of a wafer stage in a wafer stepper.
2. Description of the Related Art
During the manufacture of integrated circuits, circuit patterns for multiple chips are made on a single semiconductor wafer using techniques such as e-beam or ultraviolet photolithography. The wafer rests on a wafer stage under the control of a feedback wafer controller. The wafer stage includes a lower XY stage and an upper leveling stage. To control the leveling stage, the feedback may be measured at the surface of the wafer, or alternatively at the actuators driving the leveling stage. The first configuration introduces inaccuracies into the system because of the delay between the measurement at the wafer surface and the actuation points below the leveling stage. By measuring position at the actuators themselves, the second technique eliminates this delay, but provides an inaccurate representation of the measurement at the wafer surface.
In particular, the leveling stage driving mechanism, including the actuators and the upper leveling stage itself, exhibits nonlinear dynamics. The nonlinear effects hamper the ability of the system to quickly and accurately position the wafer stage at a desired height and keep the wafer level as it moves. Improvements in positioning and leveling would result in a higher throughput and improved exposure image quality.
FIG. 1 is a simplified block diagram illustrating an example of a conventional wafer scanner-stepper, such as the Nikon Model NSR 201, used in the manufacture of semiconductor chips. A radiant energy source 100, such as an ultraviolet light, is directed towards a reticle or mask 102. The light passing through the mask falls on an exposure area of a wafer 104. As a result, the area of the reticle illuminated by the light projects a corresponding pattern onto the exposure area of the wafer. The wafer 104 rests on a wafer stage 106, which moves under the control of a feedback wafer controller 108. The position of the wafer 104 is detected by a wafer position sensor 110, which can be implemented with a laser interferometer for measuring position in the XY direction and an encoder for measuring position in the vertical direction, for example.
The reticle may be held by a two-part reticle stage structure which includes a fine motion stage 112 and a coarse motion stage 114. The coarse stage motion is controlled by a coarse stage controller 116, and the fine stage motion is controlled by a fine stage controller 118. The XY position of the reticle is sensed by a reticle position sensor 120, which can be implemented by a laser interferometer, for example. The present invention may be employed with this system or with many other scanner-steppers known in the art.
FIG. 2 illustrates the wafer stage 106 in more detail. The wafer stage 106 moves the wafer 104 in three dimensions. The wafer stage 106 includes a lower XY stage 200 and an upper leveling stage 202. A wafer chuck 204 on the leveling stage 202 supports the wafer 104. Interferometer mirrors 206 mounted on the leveling stage 202 reflect light back to the sensor circuitry 110 to determine the position of the leveling stage 202 in the XY direction. Interposed between the lower stage 200 and upper stage 202 are leveling drive mechanisms or actuators 208.
As is well known in the art, the XY stage 200 carries the leveling stage 202, and thus the wafer 104, along a path in the XY plane. Typically, under control of the leveling stage 202 by three leveling mechanisms 208, the wafer is positioned to a desired height and maintained in a level position as the wafer travels. As is known in the art, each leveling drive mechanism 208 may include a motor 210 that turns a lead screw 212. The screw 212 is threaded into a wedge 214, and also coupled to an encoder 216 of sensor 110. Based upon rotation of the screw, the encoder 216 provides a measurement related to the height of a roller 218 supported by the wedge and thus related to the height of the leveling table 202.
Rotation of the screw 212 translates rotational motion of the motor 210 into translational motion of the wedge 214. The wedge 214 supports the roller 218, which has a fixed axle. As the wedge 214 moves in the XY plane, that motion is translated into orthogonal vertical motion by the roller 218 moving up or down the wedge 214. In this manner, three actuators 208 control the vertical position and leveling of the upper leveling stage 202.
The scanner-stepper operates as follows. A control computer 122 generates commands specifying the position of the wafer. In response, the wafer controller 108 causes the wafer stage 106 to move toward the desired or target position. The actual position of the wafer 104 is detected by the wafer sensor 110 and is fed back to a first adder 124. The difference between the commanded position and the sensed position is the following error of the wafer stage. The wafer controller 108 adjusts the position of the wafer stage 106 in response to this error.
Because of limitations on the resolving power of projection lenses used in the light source 100, the wafer is typically exposed to only a small area of the reticle mask 102 to maintain a high resolution. The reticle motion is synchronized with the wafer motion to expose more of the reticle to the wafer. Typically, the coarse controller 116 first moves the coarse reticle stage 114 in a coarse adjustment. The reticle sensor 120 feeds the position of the reticle to a second adder 126, which compares the sensed reticle position to the sensed wafer position. The difference is the synchronization error, which is used by the fine controller 118 to adjust the fine reticle stage 112 in order to minimize the synchronization error.
During exposure, the wafer 104 is scanned with the mask pattern at a constant velocity. Scanning is performed on a row of chip areas laid out in the Y direction. When the end of a row is reached, the control computer 122 inputs a command to step the wafer in the orthogonal X direction so that scanning may proceed on the next row. After stepping, motion in the X direction is halted and scanning continues in the reverse Y direction. As a result, the wafer is moved in a serpentine pattern. For more information on serpentine scanning, please refer to U.S. Pat. No. 4,818,885, issued to Davis, et al., which is incorporated by reference herein.